The invention relates to a method of radio direction-finding. The invention further relates to a radio direction-finding system.
A radio direction-finding system commonly used at microwave frequencies, generally to provide omnidirectional coverage in azimuth, comprises a plurality of identical antennae disposed about a common central point, the main beam axes of the antennae being disposed at regular angular intervals; for omnidirectional azimuth coverage, there are typically six antennae with their main beam axes disposed in the azimuth plane. To determine the direction from which radiation is incident on the system, the amplitudes of the two strongest signals respectively received at two adjacent antennae are compared; the direction lies between the main beam axes of the two antennae and the ratio of the amplitudes is a function of the angle between the direction of incidence and either of the axes. Such a system may be broadband if each of the antennae has approximately constant beamwidth over the operating band. An amplitude-comparison system has the advantages that it provides an indication of the direction of incidence in the plane of the antennae main beam axes (typically the azimuth plane) virtually independently of whether the source lies in that plane (i.e. if the source does not lie in the plane, the direction indicated is substantially the projection of the actual direction into that plane), and that the indication is generally unambiguous, but has the disadvantage that the accuracy is not very great, being for example .+-.5.degree..
Greater accuracy in radio direction-finding can be obtained by performing phase measurements on a rectilinear antenna array (typically forming an interferometer) including a pair of antennae with a sufficiently large spacing in terms of the operating wavelength to provide an unambiguous indication of direction. However, the direction of incidence obtainable from the plane measurements on such an array is the direction in the plane of the array and the source, so that if the source does not lie in a desired plane including the array (typically the azimuth plane), the direction indicated is not the direction as projected into the desired plane. It is known that by using two coplanar such arrays which face in different respective directions, it is possible to calculate the direction of incidence as projected into the plane of the arrays and/or the direction of incidence with respect to said plane. However, using a plurality of antenna arrays in each of which ambiguity can be fully resolved necessitates a large number of channels (antennae plus processing), making the system expensive.